In a third-generation communication system, typified by IMT-2000 (International Mobile Telecommunications-2000), it is particularly required that a downlink channel be of a large capacity, for example, a data transmission rate over 2 Mbps has been achieved with 5 MHz frequency bandwidth. However, a higher data transmission rate, a larger capacity, and lower cost are required in future communication systems. Further, it is also required that mobile terminals be of low power consumption. For example, a patent reference “Japanese Laid Open Patent Application No. 2003-259454” discloses a technique for upgrading the quality of signal transmission by improving the channel structure of a communication system.